practicals

Introduction to
OpenMP exercises
Slide 1
ECMWF

Class Exercise 1
Experiment with SCHEDULE clause
subroutine work(k,a,b,c)
real*8 a(k),b(k),c(k)
!$OMP PARALLEL DO SCHEDULE(STATIC) PRIVATE(I)
do i=1,k
c(i)=a(i)*exp(b(i))
enddo
!$OMP END PARALLEL DO
return
end
Use files omptest7.F and
ompjob7
Try …
(STATIC)
(DYNAMIC)
(DYNAMIC,100)
(DYNAMIC,1000)
(GUIDED)
Which is the best
for this loop?
Slide 2
ECMWF

Class Exercise 2
subroutine work(k,A,UNIQUE)
integer*8 k,A(k),unique
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
! Calculate how many integers in array A are !
! unique and return via integer arg UNIQUE. !
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
return
end
Use files unique.F and uniquejob
Slide 3
ECMWF

Class Exercise 2 (one solution)
subroutine work(k,a,unique)
integer*8 k,a(k),b(k),unique
b(:)=0
!$OMP PARALLEL DO PRIVATE(I)
do i=1,k
b(a(i))=1
enddo
!$OMP END PARALLEL DO
unique=sum(b)
return
end
Slide 4
ECMWF

Class Exercise 2 (recommended solution)
subroutine work(k,a,unique)
integer*8 k,a(k),unique
integer*4,allocatable :: b(:)
allocate(b(k))
b(:)=0
!$OMP PARALLEL DO PRIVATE(I)
do i=1,k
b(a(i))=1
enddo
!$OMP END PARALLEL DO
unique=sum(b)
deallocate(b)
return
end
Slide 5
ECMWF

Class Exercise 3
The program md.F implements a simple molecular dynamics
simulation in continuous real space. The velocity Verlet algorithm
is used to implement the time stepping. The force and energy
computations can be performed in parallel, as can the time
integration. No knowledge of the application or science involved
are required – the above was just to scare you)
3.1 Use OpenMP directives to parallelise the time integration loop
3.2 Use OpenMP directives to parallelise the computation of forces
and energies loop nest
Use files md.F and mdjob
Slide 6
ECMWF

Class Exercise 3.1 (md.F)
! The time integration is fully parallel
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
! SAFE TO ADD OPENMP DIRECTIVES FOR THIS LOOP !
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
do i = 1,np
do j = 1,nd
pos(j,i) = pos(j,i) + vel(j,i)*dt + 0.5*dt*dt*a(j,i)
vel(j,i) = vel(j,i) + 0.5*dt*(f(j,i)*rmass + a(j,i))
a(j,i) = f(j,i)*rmass
enddo
enddo
Slide 7
ECMWF

Class Exercise 3.2 (md.F)
! The computation of forces and energies is fully parallel.
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
! SAFE TO ADD OPENMP DIRECTIVES FOR THIS LOOP !
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
do i=1,np
! compute potential energy and forces
f(1:nd,i) = 0.0
do j=1,np
if (i .ne. j) then
call dist(nd,box,pos(1,i),pos(1,j),rij,d)
pot = pot + 0.5*v(d)
do k=1,nd
f(k,i) = f(k,i) - rij(k)*dv(d)/d
enddo
endif
enddo
! compute kinetic energy
kin = kin + dotr8(nd,vel(1,i),vel(1,i))
enddo
Slide 8
ECMWF

3.1 solution
!$omp parallel do private(i,j)
do i = 1,np
do j = 1,nd
pos(j,i) = pos(j,i) + vel(j,i)*dt + 0.5*dt*dt*a(j,i)
vel(j,i) = vel(j,i) + 0.5*dt*(f(j,i)*rmass + a(j,i))
a(j,i) = f(j,i)*rmass
enddo
enddo
!$omp end parallel do
Slide 9
ECMWF

3.2 solution
!$omp parallel do private(i,j,k,rij,d)&
!$omp& reduction(+ : pot, kin)
do i=1,np
! compute potential energy and forces
f(1:nd,i) = 0.0
do j=1,np
if (i .ne. j) then
call dist(nd,box,pos(1,i),pos(1,j),rij,d)
pot = pot + 0.5*v(d)
do k=1,nd
f(k,i) = f(k,i) - rij(k)*dv(d)/d
enddo
endif
enddo
! compute kinetic energy
kin = kin + dotr8(nd,vel(1,i),vel(1,i))
enddo
!$omp end parallel do
Slide 10
ECMWF